Method for producing a part from steel sheet

ABSTRACT

STEEL HAVING 0.04 WT. PERCENT CARBON AND 0.25-1.00 WT. PERCENT MANAGANESE IS PROVIDED WITH A STRENGTHENING ADDITION OF EITHER (1) 0.008 WT. PERCENT NITROGEN OR (I) SAID NITROGEN CONTENT PLUS 0.05-0.25 WT. PERCENT SILICON. THE STEEL IS ROLLED INTO A COIL OF ROLLED STEEL SHEET HAVNG A YIELD STRENGTH OF ABOUT 40,000-50,000 P.S.I. A STEEL PART IS FORMED FROM THE SHEET IN A FORMING OPERATION WHICH INCREASES THE YIELD STRENGTH TO ABOUT 60,000-65,000 P.S.I.; AND THE STEEL PART IS THEN AGE HARDENED AT AN ELEVATED TEMPERATURE TO INCREASE THE YIELD STRENGTH TO ABOUT 65,00075,000 P.S.I. AN OPTIONAL HEAVY TEMPER ROLL IS PERFORMED WHEN THE FORMING OPERATION IS RELATIVELY MILD.

United States Patent O 3,673,009 METHOD FOR PRODUCING A PART FROM STEELSHEET Bernard S. Levy, Chicago, Ill., assignor to Inland Steel Company,Chicago, Ill.

No Drawing. Continuation-impart of application Ser. No.

743,295, July 9, 1968. This application Dec. 17, Ser. No. 885,979

Int. Cl. C21c 9/46 US. Cl. 148-123 11 Claims ABSTRACT OF THE DISCLOSURESteel having 0.04 wt. percent carbon and 0.25-1.00 percent manganese isprovided with a strengthening addition of either (1) 0.008 Wt. percentnitrogen or (2) sa d nitrogen content plus 0.05-0.25 wt. percentsilicon. The steel is rolled into a coil of rolled steel sheet having ayield strength of about 40,000-50,000 p.s.i. A steel part is formed fromthe sheet in a forming operation which increases the yield strength toabout 60,000-65,000 p.s.i.; and the steel part is then age hardened atan elevated temperature to increase the yield strength to about 65,000-75,000 p.s.i. An optional heavy temper roll is performed when theforming operation is relatively mild.

CROSS REFERENCE TO RELATED APPLICATION This is a continuation in part ofapplication Ser. No. 743,295 filed July 9, 1968, and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to a methodfor forming, from steel sheet, either hot rolled or cold rolled,galvanized or uncoated, a part which, after forming and ageing, has arelatively high yield strength between about 65,000 and 75,000 p.s.i.

As used herein, the term sheet includes strip. Steel sheet is made by arolling operation performed at a rolling mill which ships the sheet to afabricator customer forming into a part. During forming, the yieldstrength of the steel is increased.

A conventional way of assuring that a formed steel part has a desiredyield strength of about 65,000-75,000 p.s.i. is to form the part fromsteel sheet having this desired yield strength. However, forming partsfrom steel sheet of this yield strength is difficult because therelatively high yield strength is accompanied by a relatively lowductility.

Steels of relatively high ductility, such as A.I.S.I. 1008 steelconventionally used for steel sheets subjected to forming operations,have a relatively low yield strength. As rolled, A.I.S.I. 1008 steelsheet has a yield strength of 25,00040,000 p.s.i. depending upon whetherit is cold rolled and annealed (25,000-30,000 p.s.i.) or hot rolled, thehot rolled giving the higher yield strength; and even should the yieldstrength of A.I.S.I. 1008 steel sheet be increased by the formingoperation, and by age hardening, the resulting yield strength of thepart is still too low.

Steel sheet which has been strained (as by forming) age hardens, andthis increases its yield strength. The full efiect of age hardening atambient temperatures would accrue in 80 to 90 days. Products formed fromsteel sheets are conventionally subjected to a painting operationfollowed by a curing operation at elevated temperatures, e.g., 350 F.,for about thirty minutes and this accelerates the age hardening of thesteel, which in turn accelerates the accompanying increase in yieldstrength.

After forming and accelerated ageing due to the curing operation, aproduct formed from conventional A.I.S.I.

Patented June 27, 1972 1008 steel sheet, cold rolled and annealed or hotrolled, has a yield strength of about 50,000-60,000 p.s.i., the hotrolled being higher. This yield strength is 10,000- 20,000 p.s.i. lowerthan that desired.

Because age hardening decreases ductility and makes forming morediflicult, it has been conventional practice to minimize age hardeningcharacteristics in steel sheet intended for forming.

SUMMARY OF THE INVENTION A method in accordance with the presentinvention purposely relies on the increase in yield strength resultingfrom forming and from age hardening at an elevated temperature toachieve the desired yield strength in a part formed from the steelsheet. However, at the time the steel sheet undergoes forming, the yieldstrength is sufiiciently low and the ductility is sufiiciently high, topermit forming without difiiculty.

A steel part formed in accordance with the present invention will have ayield strength, at the conclusion of forming (10% deformation) and agehardening operations, of between about 65,000 and 75,000 p.s.i. Theincrease in potential yield strength over that of a part made fromA.I.S.I. 1008 steel (50,000-60,000 p.s.i.) accrues from both the rollingoperation conducted at the mill and from the forming operation conductedby the customer. In addition, the incremental potential increase inyield strength, resulting from age hardening during a curing operation,remains the same as with the conventional A.I.S.I. 1008 steel; so thatno part of the net increase in potential yield strength over the 1008steel, accruing from the rolling and forming operations, is lost duringthe age hardening operation.

These improvements in yield strength are obtained by adding to a basesteel, corresponding essentially to an A.I.S.I. 1008 steel, astrengthening addition consisting essentially of (1) 0.0080.015 wt.percent nitrogen or (2) said nitrogen content plus 0.050.25 wt. percent.The silicon and the nitrogen each impart additional yield strength tothe base steel during both the rolling operation and the formingoperation. In addition, the silicon minimizes the tendency of a productformed from the steel to undergo a gradual decrease in tensileproperties over a period of years.

The composition described in the preceding paragraph provides a rolledsheet having higher than usual initial yield strength together withsufficient ductility for forming, maximum work-hardening properties anddesired age hardening characteristics.

Finishing procedures, during the hot rolling operation, and heattreating procedures, following the cold rolling operation, must also becontrolled to impart the desired properties to the steel sheet.

Other features and advantages are inherent in the composition and methodclaimed and disclosed or will become apparent to those skilled in theart from the following detailed description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A steel sheet inaccordance with the present invention has a composition essentiallywithin the following ranges:

Other elements normally present in steel in residual amounts, e.g.sulphur and phosphorus, may also be present in residual amounts.

When the steel is teemed into ingots, rimming is avoided but withoutadding aluminum or titanium.

A steel having a composition in the range outlined above may be rolledinto sheets by a hot rolling or cold rolling operation. If the sheet ishot rolled, it may, optionally, also be pickled and/or temper rolled.

The hot rolling operation is essentially conventional up to thefinishing step which is conducted at a finishing temperature in therange 14501600 (1520 F. preferable). The finished hot rolled strip isthen coiled ata coiling temperature in the range 950-1150 F. (1050 F.preferable). Hot rolled steel sheet finished and coiled as describedabove has an average ferrite grain size in the range 7-11 and a yieldstrength of about 45,00050,000 p.s.i.; and this hot rolled steel sheet,after cooling to room temperautre and pickling, is used as the startingmaterial for producing a cold rolled steel sheet in accordance with thepresent invention.

The cold rolling operation is essentially conventional, but, in order toobtain the above described properties,

in accordance with the present invention the cold rolling operation maybe followed by a heat treating operation of the continuous normalizingtype rather than batch annealing as on 1008 steel. The continuousnormalizing operation comprises heating the cold rolled steel sheet at asheet temperature in the range 15502000 F. -for a time in the range offive seconds to five minutes followed by a cooling step in which thesheet undergoes cooling down to a temperature of 200 F. at an averagecooling rate in the range 200 F. per minute to 2000 'F. per minute. In apreferable embodiment, the cold rolled steel sheet is heated at a sheettemperature in the range 1750-1850 F. for ten to fifteen seconds andcooled at an average cooling rate of 800 F. per minute.

Following the continuous normalizing operation, the cold rolled sheet issubjected to a relatively mild temper rolling step. A cold rolled sheet,prepared as described above, has a ferrite grain size in the range 6-11and a yield strength of 40,000-45,000 p.s.i.

Optionally, the cold rolled steel sheet may be box annealed at atemperature in the range of about 1035 1085 F. for 6-12 hours and thenfurnace cooled, for example.

- Unless specified to the contrary, as used subsequently herein, theterm cold rolled refers to a steel sheet which has been not onlydeformed by cold rolling but also subjected to the above describedcontinuous normalizing step or a batch annealing step and to a temperrolling step. As used herein, the term annealing, when used inconnection with cold rolled steel sheet, includes both batch annealingand continuous normalizing.

Either the hot rolled or cold rolled steel sheet may be galvanized. Whenproducing galvanized cold rolled sheet, the continuous normalizingoperation described above should be incorporated into the galvanizingline, upstream of the conventional hot dip bath; and the step of dippingthe sheet into a bath of molten zinc should be performed during the timethe sheet is undergoing said cooling at a cooling rate in the range 200F. per minute to 2000 F. per minute. This heat treating step is alsorecommended when galvanizing the hot rolled sheet.

The yield strength of the steel sheet is slightly lower when cold rolled(40, 000-45,000 p.s.i.) than when hot rolled (45,00050,000 p.s.i.). Ineither condition, the yield strength is somewhat higher than that of aconventional A.I.S.I. 1008 steel after rolling (25,000- 30,000 p.s.i.cold rolled and annealed, and up to 40,000 p.s.i. hot rolled).

As hot or cold rolled, the steel sheet of the present invention has auniform elongation of about 16%. These a and other moderate tensileproperties render the steel formable without difficulty.

The rolled steel sheet is shipped to the customer who performs apressing, drawing or other conventional forming operation on the steelsheet. At the conclusion of a typical forming operation, e.g. 10%strain, a product made from steel sheet in accordance with the presentinvention has a yield strength of about 60,000 to 65,000 p.s.i. This isabout 10,000-20,000 p.s.i. greater than the yield strength of the rolledsheet before forming. A yield strength, after forming, in the range60,000 to 65,000 p.s.i. is at least 10,000 p.s.i. greater than the yieldstrength after the same forming operation of a product made fromA.I.S.I. 1008 steel sheet, either cold rolled and annealed or hotrolled. Part of this 10,000 p.s.i. difference is due to a net increaseaccuring from the rolling operation and part of this difference is dueto a net increase accruing from the forming operation.

Conventionally, formed steel parts are painted in a conventional mannerad then subjected to a curing operation typically conducted at about 350F. The curing operation may be performed at a temperature in the rangeof ZOO-500 F. for a time of about 7 minutes to about minutes, any curingoperation performed in this range producing essentially no difference inthe final properties of the steel.

In accordance with the present invention, forming and curing areperformed before the steel fully age hardens; and the increase in yieldstrength resulting from age hardening occurring during the curingoperation is about another 8,000 p.s.i., for example, giving a finalyield strength for a product made from steel sheet in accordance withthe present invention of about 70,000 p.s.i. The 8,000 p.s.i. increasein yield strength due to age hardening is about the same as the increaseresulting from the age hardening of conventional A.I.S.I. 1008 steelsheet.

Thus, a product made from steel sheet in accordance with the presentinvention has a net increase in yield strength accuring from both therolling and the forming operations, while maintaining the same increasein yield strength during the age hardening operation, as compared to aproduct made from conventional A.I.S.I. 1008 steel sheet.

Contributing to this net increase in yield strength are the silicon andnitrogen in the amounts indicated above.

The silicon also serves another function. There is a likelihood of thesteel undergoing a gradual decrease in tensile properties over a periodof years. The presence of the silicon minimizes this decrease.

A specific example of a preferred embodiment has the In a hot rolled,pickled and oiled condition, a steel sheet rolled from the abovecomposition had the following typical tensile properties:

Yield strength, p.s.i 50,500 Tensile strength, p.s.i 63,500 It factor.18 Uniform elongation, percent 16.4 Total elongation (in 2 in.),percent 28.4

In a cold rolled condition, the steel sheet had the following typicaltensile properties:

Yield strength, p.s.i 45,600 Tensile strength, p.s.i 64,000 It factor.20 Uniform elongation, percent 16.4 Total elongation (in 2 in.),percent 26.1

When samples of the steel sheet described above were formed strain) andthen age hardened at a series of temperatures running from 200 F. to 500F. and at a series of times at each temperature, running from 7 /2minutes to 120 minutes, the resulting yield strength was always in therange of 71,00074,000 p.s.i., and the product always remained relativelyductile having a total elongation (in 2 in.) in the range of 11-14% Thecold rolled embodiment of the steel has slightly lower strengths, asrolled, than does the hot rolled embodiment; but, after forming and agehardening, the properties of both embodiments are substantially thesame.

Similar results are obtainable using the same composition withoutsilicon.

The foregoing description assume that the forming operation performedoutside the rolling mill by the fabricator is sufficiently severe toimpart an increase in yield strength to the steel of up to about 20,000p.s.i. and at least about 10,000-15,000 p.s.i. If this forming operationis relatively mild or is severe only at one part of the steel while mildin other parts, the increase in yield strength at the mildly deformedparts of the steel will be substantially less than 10,00015,000 p.s.i.Accordingly, the final yield strength after age hardening for theseparts would be somewhat less than the yield strength aim of 70,000p.s.i.

In accordance with an embodiment of the present invention, this drawbackis overcome by subjecting the steel at the mill to a relatively heavytemper roll, e.g. 2% to 10% deformation, to make up the diiferencebetween the increase in yield strength obtainable with a relativelysevere forming operation (up to about 20,000 p.s.i. and at least about10,00015,000 p.s.i.) and the increase in yield strength actuallyobtained by the relatively mild forming operation. For example, if theyield strength, as rolled, is in the range 45,000S0,000 p.s.i. and theincrease in yield strength of most of the steel product, due to forming,is only about 5,000 p.s.i. rather than 15,000 p.s.i., the differencebetween the two (10,000 p.s.i.) is made up by subjecting the steel to arelatively heavy temper roll at the mill sufiicient to impart anadditional 10,000 p.s.i. increase in yield strength to the steel tobring the yield strength up to about 70,000 p.s.i.

Generally, for hot rolled sheet, the temper rolling operation is forincreasing the yield strength before forming by about 10,000-15,000p.s.i. minus the increase in yield strength resulting solely from theforming operation, while, for cold rolled sheet, the temper rollingoperation is for increasing the yield strength before forming by about15,00020,000 p.s.i. minus the increase in yield strength resultingsolely from the forming operation. Rolled steel sheet having thecomposition and yield strength described above and subjected torelatively heavy tempering (e.g., 48% deformations), in accordance withthe present invention, does not undergo substantial age hardening atambient temperatures.

What is claimed is:

1. A method for forming, from steel sheet, a part having a yieldstrength in the range of about 65,00075,000 p.s.i., said methodcomprising the steps of:

providing a steel consisting essentially of, in wt. percent:

0.040.18 carbon,

0.25-1.00 manganese,

a strengthening addition selected from the group consisting of (1)0.008-0.015 nitrogen and (2) said nitrogen content plus 0.05-0.25silicon,

I and a balance consisting essentially or iron;

rolling said steel to produce a coil of rolled steel sheet having ayield strength in the range of about 40,000- 50,000 p.s.i. and havingage hardening characteristics at an elevated temperature;

forming said steel part from said rolled steel sheet in a formingoperation which increases the yield strength of the steel to within therange of about 60,000- 65,000 p.s.i.;

and then age hardening said steel part at an elevated temperature toincrease the yield strength of said steel to within the range of about65,00075,000 p.s.i.

2. A method as recited in claim 1 and comprising the rfurther steps of:

temper rolling said steel sheet before said forming step to increase theyield strength of the steel by about 10,000-20,000 p.s.i. minus theincrease in yield strength resulting solely from said forming step. 3. Amethod as recited in claim 2 wherein said temper rolling step comprisesdeforming said steel sheet about 4-8% without imparting theretosubstantial ambient temperature age hardening characteristics.

4. A method as recited in claim 1 (wherein said steel is painted betweensaid forming step and said age hardening step.

5. A method as recited in claim 1 wherein said age hardening step isperformed at a temperature in the range 200-500 F. for a time in therange 7 /2120 minutes.

6. A method as recited in claim 1 wherein: said rolling step compriseshot rolling said steel to provide a coil of hot rolled steel sheethaving a yield strength in the range of about 15,000-50,000 p.s.i.;

and said steel part is formed from said steel sheet with the sheet in ahot rolled condition.

7. A method as recited in claim 6 and further comprising:

temper rolling said hot rolled steel sheet before said forming step toincrease the yield strength of the steel by about 10,000-15,000 p.s.i.minus the increase in yield strength resulting solely from said (formingstep.

8. A method as recited in claim 7 and comprising:

temper rolling said sheet to impart thereto a deformation in the nange48% without imparting thereto substantial ambient temperature agehardening characteristics.

9. A method as recited in claim 1 wherein:

said rolling step comprises cold rolling and annealing said steel toPI'OIVidC a coil of cold rolled steel sheet having a yield strength inthe range of about 40,000- 45,000 p.s.i.;

and said steel part is formed from said cold rolled steel sheet.

10. A method as recited in claim 9 and further comprising:

temper rolling said cold rolled steel sheet before said forming step toincrease the yield strength of the steel by about 15,000-20,000 p.s.i.minus the increase in yield strength resulting solely from said formingstep.

11. A method as recited in claim 10 and comprising:

temper rolling said sheet to impart thereto a deformation in the range48% without imparting thereto substantial ambient temperature agehardening characteristics.

References Cited UNITED STATES PATENTS 3,139,359 6/1964 Morgan 14812.43,247,946 4/1966 Klein 14812.3 3,320,099 5/1967 Weber l48]2.3

L. DEWAYNE RUTL'EDGE, Primary Examiner W. W. STALLARD, AssistantExaminer US. Cl. X.R. l48-1 42 22 2 3 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3 Dated June 27 1972 Inventofld)sernaro S. Levy It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 38, after "wt. percent" insert- -'silicon.-.- Column Aline 70 the permissible wt. for nitrogen should reau "U.UUc-0.0l5".

Signed and sealed this 2nd day of January 1973.

(SEAL Attest:

EDWARD M.FLETCHER,JR. I R O'BERT GOTTSCHALK Attesting OfficerCommissioner of Patents

